Ceramic oxide powders with fine particle sizes have an advantage over conventional ceramic powders in that their high surface area allows them to be densified at relatively low sintering temperatures. Their particulate nature allows them to be formed using inexpensive techniques such as dry pressing and slip casting. However, as particle size is reduced into the nanoscale range (i.e., <100 nm), the fine particle size can be problematic during ceramic processing and fabrication due to agglomeration. Agglomerates create density gradients in green ceramic compacts, resulting in inhomogeneous densification, sintering stresses and exaggerated grain growth during subsequent heat treatment.
Several methods have been demonstrated for the production of nanoscale ceramic powders, using spray pyrolysis and/or vapor condensation processes, which can result in strong aggregation of the product powder. Alternative methods, such as chemical precipitation, sol-gel, and hydrothermal synthesis processes, also result in agglomeration of the powder. Thus, suitable methods are required to achieve dispersion of nanoscale particles.
In suspension, nanoscale particles agglomerate because of short-range attractive (i.e., Van der Waals) forces. These short-range attractive forces between particles overcome the electrostatic repulsion of the electrostatic double layer that surrounds the particles. A cloud of ions and counter-ions surround the particle, creating the repulsive field. Particle-particle interactions can be manipulated by pH control. The magnitude of the particle electrostatic potential, known as the zeta potential, is controlled by the suspension pH. Increasing the zeta potential increases the repulsive force between particles. However, the effectiveness of pH adjustment is limited because adjusting pH also increases the ionic strength of the suspension. As the ionic strength of the suspension increases, the ion cloud surrounding the particle is compressed, allowing closer interparticle approach. Even at extreme values of pH, where the particle surfaces are highly charged, the compression of the ion cloud allows the particles to approach close enough for the short-range attractive forces to overcome the electrostatic repulsion, and agglomeration results.
An alternative method of dispersing ceramic powders is the addition of polymers that attach to the particle surface. The polymer coating prevents particle-particle contact, and agglomeration. Such steric hindrance methods have the disadvantage that they require complete coverage of the particle surfaces. For high surface area powders, the necessary amount of dispersant can be four times the amount of powder.
Well-dispersed nanoscale suspensions can be used in conventional slip and tape casting processes to make parts that sinter at low temperatures. Nanoscale suspensions can also be used in novel approaches, such as aerosol spraying. Functional membranes and corrosion resistant coatings can be sprayed onto substrates or parts and sintered at low temperatures. Depositing such oxide films using conventional powders requires high sintering temperatures to achieve high density. Significant interaction between the coating and the part can occur at high temperatures, in addition to grain growth; as the grains in the film grow, they push one another away, forming pinhole defects. Conventional powder particle sizes are also often near target film thicknesses, making it difficult to achieve films with good cohesion and sinterablility.
The use of suspensions of ceramic powders to produce dense and continuous coatings onto substrates using aerosol spray methods requires methods to circumvent high capillary stresses that can occur during drying. These stresses can become exceptionally high as the particle size of the ceramic particles in the deposited coating is reduced into the nanoscale regime. To avoid these stresses, modifications can be made to the starting suspension and deposited coating. The liquid/vapor interfacial energy of the solvent can be reduced, the packing density of the film can be homogenized and improved, and the strength of the interparticle bonds in the coating can be increased. Drying cracks occur during the falling rate period, where the air/solvent interface has moved into the capillaries of the coating. The adhesion of the solvent to the walls of the capillaries results in tensile forces being exerted on the film. The stress exerted can be expressed by the following formula,pR=2(γlv, cos θ)/awhere: pR=capillary pressure, γlv=liquid-vapor interfacial energy, θ=solid-liquid contact angle, and a=capillary radius of curvature. From this equation and the consideration that capillary radius is directly proportional to grain size, it is evident that a film composed of nanoscale materials will suffer large drying stresses. The drying stresses from capillary pressure can be lowered by decreasing the liquid-vapor interfacial energy, using alternative solvents (e.g., alcohols), or by modifying an aqueous solvent by the addition of surfactants. Examples of surfactants include alcohols such as octanol and butanol and anionic surfactants such as alkali sulfonates, lignosulfonates, carboxolates and phosphates. Sulfonates and phosphates can leave behind inorganic components that are detrimental to sintering and the electrical properties of the fired ceramic, but organic surfactants typically do not, and are favored for ceramic applications.
Development of a successful coating process also requires good particle packing and high green strength of the applied coating. As is well described in the art, bimodal distributions pack better than unimodal distributions in the green state. Green strength of the deposited films can be improved by the addition of binders to impart a degree of plasticity to the film during drying, thus avoiding brittle fracture. Polyvinyl alcohol and methylcellulose are examples of aqueous binder systems for use in ceramics. For nanoscale systems, short chain polymers including low molecular weight starches and proteins are candidate systems.
Solid oxide fuel cells are an excellent example of an application that requires novel coating deposition technologies. Fuel cells generate power by extracting the chemical energy of natural gas and other hydrogen containing fuels without combustion. Advantages include high efficiency and very low release of polluting gases (e.g., NOX) into the atmosphere. Of the various types of fuel cells, the solid oxide fuel cell (SOFC) offers advantages of high efficiency, low materials cost, minimal maintenance, and direct utilization of various hydrocarbon fuels without external reforming. Power is generated in a solid oxide fuel cell by the transport of oxygen ions (from air) through a ceramic electrolyte membrane where hydrogen from natural gas is consumed to form water. Although development of alternative materials continues, the same types of materials are used in most of the SOFC systems currently under development. The electrolyte membrane is a yttrium-stabilized zirconia (YSZ) ceramic, the air electrode (cathode) is a porous lanthanum strontium manganite ((La,Sr)MnO3) (LSM) ceramic, and the fuel electrode (anode) is a porous Ni-YSZ cermet. To obtain high efficiency and/or lower operating temperature, the YSZ ceramic electrolyte membrane must be dense, gas tight, and thin. This requires suitable methods for depositing electrolyte membranes as thin films onto porous electrode substrates (either the cathode or the anode).
Siemens-Westinghouse is developing tubular SOFC systems based on a porous ceramic tube with a deposited YSZ electrolyte coating, and subsequently deposited anode and interconnect coatings. These systems use electrochemical vapor deposition (EVD) to deposit 40 μm thick films of YSZ onto porous LSM cathode tubes (see A. O. Isenberg, U.S. Pat. No. 5,989,634). Gaseous zirconium and yttrium precursors are pumped through a porous LSM tube sealed within a high-temperature, high-pressure enclosure. The gaseous precursors diffuse through the pores in the LSM tube and react with air to form a dense YSZ film on the outer surface of the LSM tube. EVD creates extremely dense and high quality films. However, EVD is a batch process, and difficult to scale up. The EVD process also is capital intensive, requiring a substantial amount of highly specialized equipment and operators.
Several alternative lower cost electrolyte deposition methods, including plasma-spray, sol-gel, and colloidal deposition have been proposed and are at various stages of development. Of these, progress has been made with plasma-spray methods, although cost is still relatively high. Sol-gel methods have not been entirely successful due to difficulties in depositing films onto porous substrates and inherent film thickness limitations. Colloidal deposition methods, involving deposition of ceramic coatings by aerosol spraying or dip coating methods, with subsequent coating densification by sintering, provide inexpensive alternative routes to preparation of dense electrolyte films. The approach previously has been applied to the fabrication of electrolyte films on presintered electrode substrates that do not shrink during sintering of the coating. Prior to the present invention, it has been difficult to achieve dense electrolyte coatings of reasonable thickness (i.e., greater than a few microns) on presintered substrates, because of crack formation during the sintering step. These cracks are caused by excessive shrinkage of the coating during sintering because green densities of the deposited films are relatively low. Multiple coating deposition and sintering cycles (as many as ten coating/annealing cycles) have been applied to achieve leak tight electrolyte coatings (see for example: K. Eguchi, T. Setoguchi, S. Tamura, and H. Arai, Science and Technology of Zirconia V, pages 694-704, 1993).
An alternative to depositing electrolyte films on presintered and non-shrinking substrates is to deposit the films onto electrode substrates that do shrink during sintering of the coating (see for example: G. Blass, D. Mans, G. Bollig, R. Förthmann, and H. P. Buchkremer, U.S. Pat. No. 6,066,364; and J. W. Kim, K. Z. Fung, and A. V. Virkar, U.S. Pat. No. 6,228,521). The fabrication of dense YSZ electrolyte coatings on porous anode (NiO/YSZ) substrates has been demonstrated using colloidal deposition and co-sintering methods. With this process, the green electrolyte coating is applied from a suspension onto a partially sintered and highly porous anode substrate and the bi-layer structure is then sintered at high temperature (typically 1400° C.). Both the substrate and coating shrink during sintering, so that cracking can be avoided and dense and leak tight electrolyte films can be produced. In these previous demonstrations of colloidal deposition processes, coating suspensions typically were produced by extensive milling of YSZ powder in a nonaqueous solvent, followed by sedimentation to remove coarse YSZ particles. The primary disadvantage of these previous approaches is high cost due to the poor yield during production of electrolyte suspensions and use of a nonaqueous solvent. A further disadvantage of these previous processes is that the colloidal YSZ suspensions have particle sizes that are larger than about 300 nm and the particulate YSZ material has relatively low surface areas (less than 20 m2/gram), which results in the need for high sintering temperature (1400° C.) to densify the coatings. With such high sintering temperatures, YSZ would react adversely with the LSM cathode material during co-sintering, and the co-sintered cathode/electrolyte element would exhibit poor electrochemical performance. Thus, for the most part, the previous electrolyte coating processes can only be applied to anode substrates. There are certain advantages of depositing the electrolyte films onto porous LSM cathode substrates prior to co-sintering, which is difficult to do with existing coating methods that require high sintering temperatures. For example, raw materials cost of cathode-supported SOFC plates would be lower than those of anode-supported SOFC plates. Further, one would expect improved reliability of cathode-supported SOFC plates, due to better thermal expansion match between LSM cathode and YSZ electrolyte material, and due to failures of anode-supported plates that are associated with reduction of nickel oxide to nickel metal prior to operation (and due to the undesired re-oxidation of nickel metal to nickel oxide that can occur during shut-down after operation).
There are also advantages of applying interlayer films between the porous support electrode plate (either the LSM cathode or the NiO/YSZ anode) and the deposited electrolyte (YSZ) film. The purpose of such interlayer films could be either to increase performance (e.g., by incorporating catalytic materials that enhance electrochemical reactions or by locally reducing the size of particles and pores so that the density of electrochemical reaction sites is increased), or to prevent adverse chemical reactions between the support electrode and deposited film during sintering or co-sintering. A good example of interlayer materials include lanthanide doped cerium oxide ceramic electrolyte materials, and mixtures of ceria-based electrolytes with other materials (such as catalytic metals for anode interlayer films, and/or praseodymium manganite based perovskite ceramics for cathode interlayer films). Accordingly, there is a need in the art for a lower cost process for colloidal deposition of dense coatings of a ceramic electrolyte material (e.g., YSZ) onto porous substrates of a ceramic electrode material (either the LSM cathode or NiO/YSZ anode) that are either presintered, partially sintered, or unsintered, and particularly a method that utilizes an aqueous coating suspension prepared with high yield, and that provides a deposited coating that can be densified with a low sintering temperature (1400° C. or lower).